Arab separatist al-Ahvaziya group claims responsibility for attack that killed 29 people, including women and children.

Iranian officials have blamed Gulf states and the United States for Saturday's attack on a military parade that killed 29 people, and accused them of backing the Arab separatist 'al-Ahvaziya' armed group, which claimed responsibility for the killings.

President Hassan Rouhani vowed to deliver a "crushing response", while Iran's Supreme Leader Ayatollah Ali Khamenei linked the attack with the United States and its "allies in the region".

"This crime is a continuation of the plots of the regional states that are puppets of the United States, and their goal is to create insecurity in our dear country," Khamenei said in a statement.

Iranian Foreign Minister Mohammad Javad Zarif said "regional terror sponsors" were responsible for the attack, adding he held "their US masters accountable".

"Terrorists recruited, trained, armed & paid by a foreign regime have attacked Ahvaz," Zarif said in a tweet, adding: "Iran holds regional terror sponsors and their US masters accountable for such attacks.

While he and other senior Iranian officials have not directly named the states, their comments are believed to be directed at Saudi Arabia, the UAE and Israel, which all have hostile relations with Iran and have promised to counter its influence in the region, including inside the country.

The attack in Ahvaz came as the country marked the anniversary of the start of its 1980-1988 war with Saddam Hussein's Iraq.

Four gunmen dressed in military uniform sprayed bullets into a crowd of marching soldiers, bystanders and government officials watching from a nearby stand.

At least 29 people were killed, including women and children who had come to watch the parade.

'Attackers trained by two Gulf States'

"These terrorists... were trained and organised by two ... Gulf countries," Iranian military spokesman Abolfazl Shekarchi told the official news agency IRNA.

"They are not from Daesh [an Arabic acronym for the Islamic State of Iraq and the Levant] or other groups fighting [Iran's] Islamic system ... but they are linked to America and [Israel's intelligence agency] Mossad."

According to the Iranian Students' News Agency (ISNA), the al-Ahvaziya armed group claimed responsibility for the attack

Little is known about the group, but Islamic Revolutionary Guards Corps (IRGC) spokesman Ramezan Sharif told ISNA it is "funded by Saudi Arabia".

In a statement to the Iran International news site, a spokesman for the group was quoted as saying the attack on Saturday "was in response to the repression of Ahvazi Arabs".

"We do not have a choice but to carry out a resistance."

Yacoub Hor al-Tostari, a spokesman for the al-Ahvaziya group, an umbrella organisation of all the province's armed movements, said the Ahvaz National Resistance carried out the attack.

The attack undermined the Iranian government "on the day it wants to give a message to the world that it is powerful and in control," al-Tostari told the Associated Press news agency.

To bolster his claim, he gave details about one of the attackers that the AP could not immediately verify.

Abdulkhaleq Abdulla, an adviser to Emirati Crown Prince Mohammed bin Zayed, justified the attack writing on Twitter: "attacking a military target is not a terrorist act" and "moving the battle deeper inside Iran is a declared option".

Translation: 10 Soldiers killed in an attack on a military parade in the city of Ahvaz in southwestern Iran. The attack was on a military target and wasn't a terrorist attack. Moving the battle deeper inside Iran is a declared option and will increase during the next phase.

'Fight inside Iran'

Mostafa Koshcheshm, a Tehran-based political commentator and journalist, said that the al-Ahvaziya movement had been "nurtured, supported, and trained by Saudi Arabia".

"It's been operating for the past several years, they are looking to cut off and separate Iran's energy-rich province of Khuzestan from Iran, which is exactly what Saddam Hussein wanted to do," said Koshcheshm.

"They call themselves Arab nationalists but we know they have very intimate ties to the Mujahedin-e-Khalq," he said referring to an Iranian exiled dissident group accused of killing thousands of Iranian civilians and officials.

Ahvazi Arabs are a small minority in mainly ethnic Persian Iran but sit on top of the bulk of the country's 137 billion barrels of oil.

They are divided over whether they want independence or devolution of power within a democratic, federal Iran, but at their most ambitious, some Ahvazi groups want an independent state stretching beyond the borders of Khuzestan, which sits at the head of the strategic Gulf waterway and shares a border with Iraq.

In an interview with Saudi-owned TV channel MBC last year, Saudi Crown Prince Mohammad bin Salman said that the kingdom would take the fight "inside Iran."

"We won't wait for the battle to be in Saudi Arabia," he said. "Instead, we will work so that the battle is for them in Iran."

Iran summons ambassadors

Within hours of Saturday's attack, Iran summoned diplomats from Denmark, the Netherlands, and the UK, with IRNA reporting that they were "informed of Iran's strong protests over their respective countries' hosting of some members of the terrorist group".

Bahram Qasemi called on Denmark and the Netherlands to extradite the "perpetrators and their accomplices" to stand trial, IRNA said.

"It is not acceptable that the European Union does not blacklist members of these terrorist groups as long as they do not perpetrate a crime on ... European soil," Qasemi was quoted as saying.

Khuzestan, a province bordering Iraq that has a large ethnic Arab community, has seen separatist violence in the past which Iran has blamed on its regional rivals.

It was a major battleground of the 1980s war with Iraq and the province saw unrest in 2005 and 2011 but has since been largely quiet.

Kurdish rebels frequently attack military patrols on the border further north, but attacks on government targets in major cities are rare.

On June 7, 2017, in Tehran, 17 people were killed and dozens wounded in simultaneous attacks on the parliament and on the tomb of revolutionary leader Ayatollah Ruhollah Khomeini, the first inside Iran claimed by ISIL.